The invention relates to a method for producing organic electroluminescent components having a structured electrode, in particular displays having a structured metal electrode.
With an increasing interchange of data and information, their visualization, particularly in communications technology terminals, is becoming ever more important. That information presentation normally takes place through the use of pixel matrix display apparatuses which may have additional, permanently specified symbol displays (xe2x80x9ciconsxe2x80x9d). Known technologies of pixel matrix apparatuses are, for example, cathode ray tubes although, due to space requirement, electrical power consumption and weight, they are not suitable for use in mobile and portable electronic equipment. Flat screens (xe2x80x9cflat panel displaysxe2x80x9d) are considerably more suitable for that purpose and, nowadays, are predominantly based on liquid crystal display (LC display) technology.
Simple, monochrome, passive-matrix-operated LC displays offer the advantage of cost-effective producibility as well as low electrical power consumption, low weight and small space requirement. However, the use of such displays is also linked to serious disadvantages. Specifically, the displays are not self-emitting due to the technological/physical principle, that is to say they can only be read and identified reliably in particularly favorable ambient lighting conditions. A further major constraint is the severely limited viewing angle of the display.
The problem of the lack of contrast in ambient lighting conditions that are not optimal can admittedly be improved by additionally fitting back-lighting, but that improvement is linked to a number of disadvantages. For example, back-lighting greatly increases the thickness of the LC flat screen. Specifically, while LC displays without back-lighting can be manufactured with a thickness of  less than 1 mm, the total thickness of back-lit LC displays is typically several millimeters. In addition to the lamps or fluorescent tubes, the light-conducting plastic (xe2x80x9cdiffuserxe2x80x9d) required for homogeneous illumination of the display area in particular contributes to increasing the thickness. A further major disadvantage of back-lighting is that the great majority of the electrical power consumption is required for the lighting. Furthermore, a higher voltage which is required to operate the light sources (lamps and fluorescent tubes), is normally produced with the aid of xe2x80x9cvoltage-up convertersxe2x80x9d from batteries or rechargeable batteries.
Active matrix LC displays can achieve better performance than with LC displays driven in the passive mode. In that case, each of the three primary colors in each pixel is assigned its own thin-film transistor (TFT). However, TFT technology is very costly and stringent requirements are placed on the substrates that are used, due to the high process temperatures that occur. The price for active matrix LC displays is correspondingly high.
The switching time of a single liquid crystal pixel is typically several milliseconds due to the physical principle of reorientation of a molecule in the electrical field, and furthermore is very highly temperature-dependent. The slow and delayed formation of the image, for example in traffic equipment (navigation systems in motor vehicles), thus has a particularly disturbing effect at low temperatures. LC technology can therefore be used only to a limited extent in the case of applications in which rapidly changing information and images are displayed, for example in video applications.
Other display technologies have either not yet reached the level of maturity for technical applicability, for example flat panel CRTs (CRT=cathode ray tube), or their use, particularly in portable electronic equipment, is subject to serious disadvantages resulting from specific characteristics: high operating voltage for vacuum fluorescence displays and inorganic thin-film electroluminescent displays, and high costs for displays based on inorganic light-emitting diodes.
Those disadvantages can be overcome by using displays based on organic light-emitting diodes (OLEDs). That new technology has a large number of advantages over LC displays, of which the major advantages are as follows:
Due to the principle of self-emissivity, there is no necessity for back-lighting, which considerably reduces the space requirement, power consumption and weight.
The typical switching times of pixels are in the order of magnitude of 1 xcexcs and thus allow rapid image sequences to be displayed without any problems.
The switching times have no disturbing inertia at low temperatures.
The reading angle is considerably greater than in the case of LC displays, and is virtually 180xc2x0.
There is no need for the polarizers required for LC displays, so that increased brightness can be achieved even with a multiplexed display.
In contrast to other display technologies, organic light-emitting diodes can also be produced on flexible substrates and with non-planar geometries.
The production and layout of displays based on organic light-emitting diodes is much easier, and thus more cost-effective to achieve, than LC displays. Layout and production are typically carried out as follows.
The substrate, for example glass, is coated over the entire surface with a transparent electrode that is composed, for example, of indium tin oxide (ITO). Both the transparent electrode and the top electrode must be structured to produce pixel matrix displays. In that case, both electrodes are normally structured in the form of parallel conductor tracks, with the conductor tracks of the transparent electrode and top electrode running at right angles to one another. The structuring of the transparent electrode is carried out by a photolithographic process including wet-chemical etching methods, the details of which are known to the person skilled in the art. The resolution that can be achieved by using those methods is essentially limited by the photolithographic steps and the nature of the transparent electrode. Based on the prior art, both pixel sizes and non-emitting intermediate spaces between the pixels of a few micrometers in size can be achieved in that case. The length of the conductor tracks (which are in the form of strips) of the transparent electrode may be up to many centimeters. Depending on the lithography mask being used, emitting surfaces with a size of up to several square centimeters can also be produced. The sequence of the individual emitting surfaces may be regular (pixel matrix display) or variable symbol displays).
One or more organic layers are applied to the substrate with the structured transparent electrode. Those organic layers may be composed of polymers, oligomers, low molecular-weight compounds or mixtures thereof. Liquid-phase processes (application of a solution through the use of spin coating or wiping) are normally used for applying polymers, for example polyanline, poly(p-phenylene-vinylene) and poly(2-methoxy-5-(2xe2x80x2-ethyl)-hexyloxy-p-phenylene-vinylene), while gas-phase deposition (vapor deposition or xe2x80x9cphysical vapor depositionxe2x80x9d, PVD) is preferred for low molecular-weight and oligomer compounds. Examples of low molecular-weight compounds, which preferably transport positive charge carriers, are: N,Nxe2x80x2-bis-(3-methylphenyl)-N,Nxe2x80x2-bis-(phenyl)-benzidine (m-TPD), 4,4xe2x80x2,4xe2x80x3-tris-(N-3-methylphenyl-N-phenylamino)triphenylamine (m-MTDATA) and 4,4xe2x80x2,4xe2x80x3-tris-(carbazol-9-yl)triphenylamine (TCTA).
Hydroxyquinoline aluminum III salt (Alq) is used, for example, as an emitter, which can be doped with suitable chromophores (quinacridon derivatives, aromatic hydrocarbons etc.). Additional layers, which influence the electrooptical characteristics as well as the long-term characteristics, may also be present and may be composed, for example, of copper phthalocyanine. The total thickness of the layer sequence may be between 10 nm and 10 xcexcm, typically being in the range between 50 and 200 nm.
The top electrode is normally composed of a metal, which is in general applied by gas-phase deposition (thermal evaporation, sputtering or electron beam evaporation). Base metals, which are therefore reactive in particular with water and oxygen, are preferably used, such as lithium, magnesium, aluminum and calcium as well as alloys of those metals with one another or with other metals. The structuring of the metal electrode required for producing a pixel matrix configuration is in general achieved by applying the metal using a shadowmask having appropriately shaped openings.
An OLED display which is produced in that way may include additional devices that influence the electrooptical characteristics, such as UV filters, polarization filters, anti-reflection coatings, devices known as xe2x80x9cmicro-cavitiesxe2x80x9d as well as color-conversion and color-correction filters.
Furthermore, hermetic packaging is provided, through the use of which the organic electroluminescent displays are protected against environmental influences, such as moisture and mechanical stresses. In addition, thin-film transistors may be present in order to drive the individual pixels.
In the case of high-resolution displays, which make it possible to present a large amount of information, fine structuring of the metal electrode in the form of conductor tracks is required, that is to say both the width of the conductor tracks as well as the intermediate spaces must be capable of being structured in the xcexcm range while maintaining tight tolerances. The width of a conductor track in that case may be between 10 xcexcm and several hundred micrometers, preferably between 100 and 300 xcexcm. In order to achieve a high filling factor (ratio of the active, light-emitting area to the total area of the display configuration), it is also necessary for the intermediate spaces between the metallic conductor tracks as well as the intermediate spaces between the conductor tracks of the transparent electrode to be only a few micrometers.
In order to produce structured metal electrodes, a metal layer can first of all be applied to the entire surface through the use of a gas-phase process, and that layer is then structured using photolithographic methods. However, that has the major disadvantage of possibly damaging the sensitive, organic functional layers, when those parts of the metal layer which are not required (xe2x80x9cintermediate spacesxe2x80x9d) are removed. Such processes for defined removal of parts of the metal layer are, for example, lift-off processes and plasma etching. However, since even damage to or destruction of individual pixels leads to a considerable adverse effect, even to the extent of the entire display becoming unusable, reliable process control and thus a high yield in production cannot be achieved with those techniques. A further exacerbating factor is that the use of both aqueous and organic solvents for removal of a part of the metal layer, by destructive diffusion of the solvent into the organic functional layers, is only feasible to an extremely limited extent.
A further option for structuring of the metal electrode is selective deposit-on of the metal, for example in the form of conductor tracks, through the use of appropriately structured shadowmasks. Those shadowmasks may be produced from stainless steel or from glass which can be structured photo-lithographically. The intermediate spaces between individual conductor tracks in that case correspond on the side of the shadowmask to thin xe2x80x9cwebsxe2x80x9d, and the actual conductor tracks of the metal electrodes are deposited through appropriately shaped openings. Both for reasons of stability of the shadowmask and for production-engineering reasons during manufacture, web widths of  less than 150 xcexcm for stainless steel and  less than 90 xcexcm for glass cannot be produced reliably. A further exacerbating factor is that, during the process of depositing the metal, the distance between the substrate and the mask must be no more than a few micrometers, in order to ensure that the metallic structures have sharp edges. Furthermore, the shadowmask must not damage the organic functional layers by excessive contact pressure against the substrate. In addition, the production of non-planar organic light-emitting diodes on curved surfaces is made substantially more difficult as a result of those problems. A further problem with respect the producibility of organic LED displays using shadowmasks is that the substrate, with the organic functional layers, must be positioned reproducibly and with a high throughput (production of several thousand displays per day) to an accuracy of a few micrometers above the associated structures of the shadowmask. Furthermore, for geometric reasons, it is impossible to use shadowmasks to represent shapes or symbols in which a non-emitting area is completely enclosed by an emitting area, for example concentric circles.
U.S. Pat. No. 5,276,380 discloses electroluminescent displays in which vertical xe2x80x9cwallsxe2x80x9d are disposed between the light-emitting pixel rows and overhang the organic electro-luminescent medium. Such a construction has the disadvantage of not being suitable for producing symbols of any desired shape. In addition, the size of the individual pixels is limited by the height of the walls. Large-area displays therefore cannot be produced in that way.
It is accordingly an object of the invention to provide a method for producing organic electroluminescent components, which overcomes the hereinafore-mentioned disadvantages of the heretofore-known methods of this general type and which allows structured electrodes to be produced in organic electro-luminescent components in a manner that is suitable for manufacture, in particular the production of finely structured metallic top electrodes for high-resolution displays and for presentations that include finely structured details, in which case it is intended to be possible to simultaneously present separate symbols and information on the display area that is broken down into pixels.
With the foregoing and other objects in view there is provided, in accordance with the invention, a method for producing organic electroluminescent components having a structured electrode, in particular displays having a structured metal electrode, which comprises applying at least a first and a second layer onto a bottom electrode located on a substrate, with a defined boundary remaining between the first and second layers, the first layer being electrically insulating and not damaged when applying the second layer, the first layer having a higher solubility rate in a liquid developer than the second layer, and the second layer being able to be structured; structuring the second layer and transferring the structure to the first layer; applying at least one organic functional layer onto the second layer; and depositing a top electrode onto the organic functional layer.
The invention provides a novel method for producing organic electroluminescent components. This method allows the production of structured electrodes, such as metal electrodes, in particular for organic electroluminescent displays, without there being any risk of damage to the organic functional layers, as is the case with known processes. Furthermore, it is possible to dispense with the use of shadow masks, which are susceptible to faults, and the achievable resolution is considerably improved with respect to the lower limit and bandwidth.
The top electrode, which preferably has a low work function for electrons and thus functions as an electron-injecting electrode, is composed, in particular, of metal or a metallic alloy. However, this electrode may also have a layer structure, in which case a metal or ITO layer is disposed as a (transparent) electrode on a thin dielectric layer ( less than 5 nm) that is composed, for example, of lithium fluoride or aluminum oxide.
The essential feature of the method according to the invention is that the first layer which is applied onto the bottom electrode (that can be structured), that is to say the lower layer, is not damaged when the second layer (upper layer) is applied, and a defined boundary is maintained between the two layers.
In accordance with another mode of the invention, the first and/or the second layer is or are advantageously composed of an organic film-forming material, preferably of a photoresist.
Photoresists are light-sensitive film-forming materials having a solubility behavior which changes as a result of illumination or irradiation in this case, a distinction is drawn between positive and negative photoresists. If, in the present case, both the upper layer and the lower layer are composed of a photoresist and if the two photoresists are sensitive in the same wavelength band, then the photoresist of the lower layer must not be a negatively operating system.
In accordance with a further mode of the invention, an essential feature of a preferred embodiment includes a photolithographic process in which at least two layers are applied to the transparent bottom electrode, possibly after it has been structured, the first layer of which is composed of a varnish or a positive photoresist and the second layer of which is composed of a positive or negative photoresist. In the case of a first layer composed of photoresist, that layer is floodlit before the application of the second layer. The layers are then structured in such a manner that the organic functional layers and the (metallic) top electrode can be applied or deposited over the area thereon. The structuring of the layers and of the top electrode in this case runs transversely with respect to the structuring of the bottom electrode. The application of the organic functional layer or layers onto the second layer in general can be carried out both through the use of a thermal evaporation process as well as from a solution, for example by centrifuging on or wiping, and subsequent drying.
In the case of the photolithographic method step, the following is important. It must be possible to coat over the first of the two layers (xe2x80x9covercoatablexe2x80x9d). This means that the two layers can be applied one over the other without any so-called Intermixing, that is to say the varnishes which are used are soluble in different solvents, so that the xe2x80x9cphotoresistxe2x80x9d varnish in the first layer is not attacked by the solvent for the photoresist in the second layer. This ensures that the defined structure of the first layer is retained when the second layer is applied, and that a defined boundary exists between the two layers.
It is also necessary with regard to the photolithographic method step, for the first layer to have a higher development rate than the second layer. This means that, when the developer solution is applied to the varnish layers for the treatment required, after exposure, for structuring, the first layer dissolves more quickly than the second layer. It is advantageous in this case if the two layers can be treated, that is to say developed, by using the same developer which is, in particular, an aqueous-alkaline developer.
In general, electrically insulating, organic or inorganic materials are used for the lower layer. Suitable inorganic materials are, for example, silicon dioxide, silicon nitride and aluminum oxide.
In accordance with an added mode of the invention, alternatively, the lower layer is composed of non-photosensitive polyimide which can be developed by alkaline techniques.
In accordance with an additional mode of the invention, the lower layer is photosensitive and in this case is preferably composed of a positive photoresist, based on polyglutarimide or polybenzoxazole.
In accordance with a concomitant mode of the invention, the upper layer is likewise advantageously a photoresist. This layer is preferably composed of a positive photoresist (positive resist) based on Novolak/diazoquinone, or is composed of a negative photoresist (negative resist) based on Novolak/photo-acid. Poly(methylmethacrylate) (PDMA), poly(methyliso-propylketone) (PMIPK) and so-called chemically amplified systems on a poly-(p-hydroxystyrol) base with tertiary butoxycarbonyl protection groups (t-BOC groups)can also be used as the positive resist. Poly(silphenylene-siloxanes) can also be used, for example, as the negative resist.
Alternatively, it is possible to structure the upper layer indirectly. Amorphous carbon (a-C) or amorphous carbon containing hydrogen (a-C:H) is then used, for example, as the layer material for this purpose. Such layers are structured in an oxygen plasma, with an etching mask in the form of a photoresist layer containing silicon being used, in particular a so-called CARL resist (CARL=Chemical Amplification of Resist Lines) or a TSI system (TSI=Top Surface Imaging). After structuring, the upper layer advantageously has a larger structure width than the lower layer. The difference in the structure width (xe2x80x9coverhangxe2x80x9d) in this case is advantageously between 0.1 and 50 xcexcm, in particular between 1 and 10 xcexcm. The thickness of the lower layer is preferably 0.1 to 30 xcexcm, in particular 0.5 to 10 xcexcm, and that of the upper layer is 0.1 to 30 xcexcm, in particular 0.5 to 5 xcexcm.
A major advantage of the method according to the invention is that it can be used both for pixel displays and for symbols (xe2x80x9ciconsxe2x80x9d) of any desired shape. For example, an electroluminescent display may thus be composed of a central region with individual pixels disposed in lines and columns and surrounded by a row of symbols. Further advantages include the following:
It is possible to dispense with the use of shadow masks for structuring deposition of the (metallic) top electrode. This avoids a high-cost process step that is subject to faults.
The processes required to remove a portion of the metal electrode are avoided. There is therefore no risk of the organic functional layers being damaged by plasma processes or by the influence of solvents. Furthermore, a high level of process reliability and stability are achieved, and more advantageous costs can be achieved as a result of the increase in the yield.
The metal electrode may be deposited over a large area. The parts which are deposited on the intermediate spaces between the actual conductor tracks do not need to be removed since there is no electrically conductive connection between the actual conductor tracks and the metal in the intermediate spaces.
In comparison with structuring of the metal electrode through the use of a single photoresist layer, there is a major advantage in that the achievable xe2x80x9cundercuttingxe2x80x9d in the case of a two-layer configuration is considerably greater than in the case of a configuration with only one layer. The probability of an electrical short-circuit between two adjacent metallic conductor tracks, which would lead to failure of the display, is thus considerably lower.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a method for producing organic electroluminescent components, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.